How do I define a type for a function which returns a pointer to its own type?
That is... how do I typedef fn_type below?
C++ extensions are fine to use if need be, and welcome if they make life easier for this problem.Code:fn_type * fn_ptr = start_function; while (fn_ptr != NULL) { fn_ptr = fn_ptr(); }
Last edited by QuestionC; 06-13-2007 at 08:47 AM.
Callou collei we'll code the way
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So you want a pointer to a function that returns this same type (ie. same pointer to a function)?
Are you sure you can't think of some easier method to achieve your goal?
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
I went to the link http://c-faq.com/decl/recurfuncp.html and tried to understand it.
I have a few questions:
I've written the following code, and at least it works :Code:int (*funcptr)(); states that funcptr is a pointer to a function that takes no arguments and returns an int. typedef int (*funcptr)(); does this define funcptr as a type? (this is what I think so, but I haven't used typedef like this before ) again funcptr (*ptrfuncptr)(); states that ptrfuncptr is a pointer to a function that takes no arguments and returns a funcptr. typedef funcptr (*ptrfuncptr)(); same as before : does this define ptrfuncptr as a type?
Here the function start-function is being called again and again, (the function itself supplies it's own address) though not recursively, which is obviously not possible as it supplies a pointer to itself in the return statement.Code:#include <stdio.h> typedef int (*funcptr)(); /* generic function pointer */ typedef funcptr (*ptrfuncptr)(); /* ptr to fcn returning g.f.p. */ int main() { funcptr start_function(); ptrfuncptr state = start_function; while (state != NULL) state = (ptrfuncptr)(*state)(); return 0; } funcptr start_function() { static int i=0; ++i; printf("\nIn function \"start funtion\" for the %dth time!", i); if(i==5) return NULL; else return start_function; }
Looking for some comments.
Thanks.
regards,
koodoo.
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Statutory Warning : Beware of C... it's highly addictive.
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Having the function return a void * is probably the best method. The following is valid (in C, but it wouldn't be in C++) and and least with my compiler, is free of warnings:
Code:#include <stdio.h> #include <stdlib.h> typedef void *(*rfunc)(void); void *func1(); void *func2(); void *func3(); void *func1() { printf("func1()\n"); return func2; } void *func2() { printf("func2()\n"); return func3; } void *func3() { printf("func3()\n"); return NULL; } int main() { rfunc f = func1; while(f) { f = f(); } return 0; }
I hadn't noticed this originally. If you're willing to use a little C++, your problem is easily solved by creating a base class with a virtual function (call it "doIt()" or whatever) that returns a pointer to that same base class. Then you can derive various classes from the base and put whatever you want in their doIt() methods. Then you could just call polymorphically through the base class.Originally Posted by QuestionC
I'd post code, but since this is the C board that's as far as I'll go at the moment.
Well it certainly is possible. We can assume that a function always has the same address throughout the program, and since the address is returned by the called function it can be assigned and dereferenced in a while statement. What's written in cases like these is not really recursive because the function completes a step every time as opposed to calling itself and bottoming out, but as stated by the article: in coding a truly recursive solution the function pointers cannot act like real functions because their type compounds with every call.Here the function start-function is being called again and again, (the function itself supplies it's own address) though not recursively, which is obviously not possible as it supplies a pointer to itself in the return statement.
I think in this case the word "recursive" is applying to the type (a recursively defined type), not the particular function call pattern.
That's debatable.
main.c: In function `func1':
main.c:13: warning: ISO C forbids return between function pointer and `void *'
main.c: In function `func2':
main.c:19: warning: ISO C forbids return between function pointer and `void *'
main.c: In function `main':
main.c:33: warning: ISO C forbids assignment between function pointer and `void *'
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
